In a civil engineering construction site or a quarry site of a mine, various work vehicles such as a dump truck and an excavator are operated. Particularly in a mine, a superjumbo work vehicle is used. Since such a work vehicle has a vehicle width, a vehicle height, and a longitudinal length which are noticeably larger than those of a general vehicle, it is difficult for an operator to check and recognize a peripheral environment by a side mirror and the like. There is proposed a device which monitors a periphery of a vehicle as a technique of assisting a driving operation by helping an operator to simply recognize a peripheral environment of a vehicle. For example, a device is disclosed which suggests a risk degree of an obstacle within an operation position range of a working unit (Patent Literature 1).